1. Field of the Invention
The present invention relates generally to fuel cell systems, and more particularly to a new and improved gas diffusion layers for use in Proton Exchange Membrane (PEM) fuel cell systems.
2. Discussion of the Related Art
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In PEM-type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements, sometimes referred to as the gas diffusion media components, that: (1) serve as current collectors for the anode and cathode; (2) contain appropriate openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts; (3) remove product water vapor or liquid water from electrode to flow field channels; (4) are thermally conductive for heat rejection; and (5) have mechanical strength. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (e.g., a stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the MEA described earlier, and each such MEA provides its increment of voltage.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
Examples of technology related to PEM and other related types of fuel cell systems can be found with reference to commonly-assigned U.S. Pat. No. 3,985,578 to Witherspoon et al.; U.S. Pat. No. 5,272,017 to Swathirajan et al.; U.S. Pat. No. 5,624,769 to Li et al.; U.S. Pat. No. 5,776,624 to Neutzler; U.S. Pat. No. 6,277,513 to Swathirajan et al.; U.S. Pat. No. 6,350,539 to Woods, III et al.; U.S. Pat. No. 6,372,376 to Fronk et al.; U.S. Pat. No. 6,521,381 to Vyas et al.; U.S. Pat. No. 6,524,736 to Sompalli et al.; U.S. Pat. No. 6,566,004 to Fly et al.; U.S. Pat. No. 6,663,994 to Fly et al.; U.S. Pat. No. 6,793,544 to Brady et al.; U.S. Pat. No. 6,794,068 to Rapaport et al.; U.S. Pat. No. 6,811,918 to Blunk et al.; U.S. Pat. No. 6,824,909 to Mathias et al.; U.S. Patent Application Publication Nos. 2005/0026012 to O'Hara; 2005/0026018 to O'Hara et al.; and 2005/0026523 to O'Hara et al., the entire specifications of all of which are expressly incorporated herein by reference.
The gas diffusion media component of a PEM fuel cell is typically comprised of a non-woven carbon fiber paper, e.g., those available from Toray Industries, Inc. (Tokyo, Japan), or a woven carbon cloth, e.g., those available from Zoltek Corporation (St. Louis, Mo.) under the PANEX trade name. Upon arrival at the PEM fuel cell manufacturer, the as-is product is typically post treated in order to render the material hydrophobic. Additionally, it has become more common practice to apply a microlayer ink (sometimes referred to as a microporous layer (MPL)) to the gas diffusion layer (GDL) for more effective water management properties.
The gas diffusion media component of the fuel cell has many functions to fulfill in order to operate successfully in a PEM fuel cell. For example, the primary tasks of the gas diffusion media component include: (1) acting a diffuser for reactant gases traveling to the electrode; (2) transporting product water to the flow field; and (3) conducting electron and transferring heat generated at MEA to the coolant.
Along with the above requirements, automotive application requirements typically demand more of the gas diffusion media component, of which the following are included: (1) protect the MEA from damage from the bipolar plates during compression; (2) freeze compatibility; and (3) durability for thousands of hours under compression.
As previously noted, conventional GDLs are typically composed of non-woven carbon fiber paper or carbon cloth with an MPL coating thereon. The MPL is a carbon black/fluorinated polymer matrix that is coating by ink onto the gas diffusion media substrate. It is believed that the microporous layer is responsible for most of the water management of the substrate/layer package. In addition, the MPL behaves as a buffer to reduce some of the high stress spots on MEA during stack compression because it is composed of fine carbon and PTFE particles. Even though good performance has been achieved with GDLs in this configuration, there are still several unresolved issues.
First, because of the nature of non-woven carbon fiber paper and carbon cloth (fiber overlay and resin binder hard spots), there are generally high stress spots imposed on the MEA during stack compression, even with MPL coating as a buffer, which is believed to be one of the causes of MEA failures.
Second, the high porosity nature of carbon fiber paper or cloth may trap water after the stack shuts down, which is typically hard to be removed through a quick purge. This can cause freeze related damage.
Third, de-lamination is another serious issue for the MPL coating carbon fiber paper or cloth. The fluorinated polymer in the MPL serves as a binder as well as a hydrophobic agent. Intrinsically, the adhesion of this layer is less than desired due to the nature of the material. Rubbing/washing off of this layer during the build or operation phases of the running cell jeopardizes durability. Furthermore, good adhesion is required for consistently high current operation and freeze capability of the cell.
CARBEL®, a gas diffusion media product, readily commercially available from W. L. Gore & Associates, Inc. (Newark, Del.), can be considered as a “stand-alone” MPL-like material, which is essentially a carbon powder filled expanded PTFE film. CARBEL® cannot typically be used alone because it lacks the stiffness required to support the MEA, especially over the gas flow channel regions. However, the fuel cell performance using simply laminated CARBEL®, carbon fiber paper or cloth is not as good as the MPL coated carbon fiber paper or cloth. In other words, CARBEL® does not fulfill the required water management needs, especially under very wet conditions.
Accordingly, there exists a need for new and improved gas diffusion layers for gas diffusion media components of PEM fuel cell systems, especially those that include an independent or stand-alone MPL which can handle the water management requirements, generate substantially uniform compression over the MEA, and still maintain acceptable carbon fiber paper like mechanical properties.